1. Field of the Invention
The present invention relates generally to the field of glass compositions, and more particularly, to alkaline resistant glass compositions suitable as fillers and modifiers.
2. Description of the Related Art
It is well known that introducing glass fibers improves the mechanical properties of many materials, such as cementitious products and various polymers. In particular, glass fibers improve the tensile strength of the composite cementitious or polymeric products. Similarly, expanded glass microparticles can be used to impart low density to cementitious and polymeric composites. Such microparticles may also have other beneficial effects, such as reducing moisture movement, thermal movement, improving thermal insulation value or improving workability. However, in a strong alkaline environment, silicate glasses are subject to rapid corrosive attack. This phenomenon has severely limited the use of glass in reinforcing concrete, which exhibits strong alkalinity during curing. For example, during the hydration of Portland cement, calcium hydroxide is formed as a reaction byproduct. In addition, alkali metal impurities are solubilized in the form of hydroxides.
Several approaches have been used to retard alkaline attack on glass in cement environments. One approach is to polymer coat the glass which, while somewhat effective at protecting the glass from corrosive attack, results in a weak mechanical bond to the surrounding cement matrix.
Another approach is to develop better alkaline resistant glass (AR glass) by altering its chemical composition. Common commercially available glass compositions that have been used for this purpose include E-glass which typically consists essentially of 54% SiO2, 14% Al2O3.0.3 Fe2O3, 17.5% CaO, 4.5% MgO, 10% B2O3, and C-glass, which consists essentially of 65.6% SiO2, 4% Al2O3.Fe2O3, 14% CaO, 3% MgO, 8% Na2O.K2O, 5.5% B2O3 and 0.5% K2O, (Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd Edition, Volume 10, 1966.) However, these glasses are susceptive to alkaline dissolution at high pH levels and are therefore not suitable as additives in many cementitious compositions. Accordingly, their use as a material enhancing additives has been primarily limited to less aggressive polymers.
Currently, alkali resistant glasses are made by adding refractory oxides such as zirconia and titania. One group of alkali resistant glasses is those containing appreciable amounts of zirconia (ZrO2). Zirconia Alkali resistant (ZAR) glass compositions generally have high zirconia content, oftentimes in the range of 15-20 wt. % or more. One example of such a commercially available glass is sold under the trade name CEM-FIL® by Vetrotex Cem-Fil S.L. in Alcalá de Henares, Spain for use as a reinforcing fiber in cement. However, the addition of zirconia raises the melting temperature of the glass composition, which greatly increases the processing cost. To counteract the negative effects of ZrO2, alkali oxides are typically added to reduce the melting temperature to a more workable level. In many cases, more than 10 wt. % alkali oxides are added to reduce the melting temperature to a more workable level of the zirconia silicate system.
The good performance of ZAR glasses in high alkaline environments is believed to be due to the relatively low solubility of Zr—O—Zr species. In some applications, titania may be added to further improve the durability of the glass. However, the addition of titania presents additional drawbacks, such as further increases in melting temperature, which increases the processing cost, and additionally increases the susceptibility of the glass to divitrification. Therefore, while the addition of titania may provide some benefits, there are associated costs in terms of materials and processing.
While available ZAR glasses have been used in fiber cement products with some success, both zirconia and titania are very expensive when compared with the cost of other raw glass materials, and therefore, the material cost prohibits this type of glass from widespread use in the cement industry. Moreover, even the expensive ZAR glasses are subject to corrosion damage in harsh alkaline solution environments, such as hydrating cement.
While tests have shown that ZAR glass exhibits improved corrosion resistance over other glass types, the corrosive effects of an alkaline solution on glass are exacerbated as the temperature is increased. The majority of accelerated durability tests on glass compositions for use in alkaline environments have been carried out at temperatures around or below 100° C., e.g. 90° C. in strong alkaline solutions. For example, ASTM C 1203-91 specifies a test method for quantitative determination of alkali resistance of ceramic-glass enamel based on the weight loss in 10 wt. % NaOH in water. The test duration is 2 hours and the temperature is 95° C. The international tests ISO 695-1991 and DIN 52322 both make use of a mixture of equal volumes of 1 M NaOH and 0.5 M Na2CO3 at a temperature of 102.5° C. for 3 hours.
However, certain cementitious products are rapidly cured at temperatures well beyond 100° C. In fact, curing temperatures of high performance fiber cement products can be as high as 180° C., or even higher under high temperature hydrothermal conditions. It has been noted that in strong alkaline solutions, the rate of corrosion attack commonly doubles for each 10° K increase in temperature.
Harsh curing conditions are normally experienced when green cement products are cured in an autoclave, and cement inclusions are exposed to pH levels typically within the range of 12.5 to 14, and temperatures can reach as high as 180° C. or higher. In such an aggressive alkali environment, the glassy materials must possess even higher chemical durability in order to withstand excessive dissolution in the high temperature cementitious matrix. Dissolution is not desirable, since it not only degrades the mechanical integrity of the composite where the glassy materials form the inclusions, but it may change the chemistry of the cementitious region in the immediate vicinity of the glassy materials. Both of these results reduce the quality of the cementitious composites.
Further, polymeric materials are known to be reinforced with glass fiber or glass mesh and exposed to alkaline environments. While the polymers themselves may resist corrosive attack, the embedded glass materials may still be susceptible to corrosive attack.
Accordingly, there remains a need for an improved glass which is highly resistant to the corrosive effects of basic environments, including but not limited to alkaline environments. Further, there is a need for an improved glass that is highly resistant to an alkaline environment at elevated temperatures beyond 100° C. Additionally, the amount of fibers typically used to reinforce cement and polymers is quite large, even reaching 20% or more of the total cement or polymer composite by weight. Therefore, it is particularly important that glass fibers or materials made for cementitious and polymeric applications are manufactured economically.